One handy feature that I would like to see is for Tableau Online to go to the next step and expand their data driven alerts to be sent to a cell phone number. An example where this could be done is adding in additional data fields for users with their cell number, carrier, and country. Then adding where you want the Alert to be sent to (e-mail/text/both). This would naturally have to be a truncated version that are currently sent out. But, this would be handy to alert users that metrics have meet, exceeded, or fell below the target that has been sent.
The other, as a site admin, would be to get a text message alert for when an extract failed. Again, something short and sweet with the extract that failed, date/time, and maybe the reason why.